The present invention relates to an imaging zoom lens system, and in particular to a compact zoom lens system of the three-unit type that has an F number range of about 1:4 to 1:5.6 and a viewing angle in an approximate range of .+-.6.degree. to .+-.18.degree..
Prior art zoom lens systems that satisfy viewing angle and F number specifications of about .+-.6.degree. to .+-.18.degree. and about 1:4 to 1:5.6, respectively, are disclosed in Unexamined Published Japanese patent applications Nos. 12614/81 and 126819/81.
Each of these systems is generally referred to as a "four-unit type" zoom lens system, and the respective lens units responsible for four different functions, namely, focusing, zooming, compensation for defocusing, i.e., offset in focal point introduced as a result of zooming, and relaying of the image. This lens composition has the advantage that the overall lens length does not change during zooming since only the second and fourth units need to be moved to achieve zooming. On the other hand, this composition suffers from the disadvantage that, in order to shorten the overall lens length, the refractive powers of the individual lens units have to be increased, which results in less than satisfactory compensation for various aberrations.